1. Field of the Invention
The present invention relates to a mobile communication system, a base station apparatus and a method of controlling the mobile communication system, and, more particularly, to a mobile communication system in which a combination of three methods: code division multiple access (CDMA), time division multiple access (TDMA) and time division duplex (TDD) is used as an access method, and in which communication is performed by using one frequency in two cells adjacent to each other or a frequency with a possibility of interference, a base station apparatus adapted to the mobile communication system and a method of controlling the mobile communication system.
2. Description of the Related Art
In a CDMA system, a wireless resource called a shared channel is defined. This shared channel is used mainly for packet users in such a manner that one channel is shared by a plurality of users. Over this shared channel, transmission is performed at different times in correspondence with the users and call setting control can be easily performed. However, there is a need for scheduling for determining which user is allowed to use the resource.
FIG. 9 shows an example of a frame configuration of a CDMA/TDMA/TDD method. In the example shown in FIG. 9, one frame is constituted by fifteen time slots, and eight codes at the maximum can be multiplexed. In the CDMA/TDMA/TDD method used as a wireless access method, wireless resource assignment with respect to a traffic will be as described below.
(1) The TDD method is one of methods in which a downstream channel and an upstream channel are separated, and in which use of one frequency is divided with respect to time. According to the TDD method, therefore, the fifteen time slots are allotted between upstream channels and downstream channels.
(2) The TDMA method is a multiple access method in which a time period for transmission of one frame using one frequency is divided into a certain number of time slots and users use different time slots assigned from the divided time slots. According to the TDMA method, therefore, one or more of fifteen time slots are assigned to one user.
(3) The CDMA method is a multiple access method in which wireless channels are set by using different codes in correspondence with users at one radio frequency. According to the CDMA method, therefore, user multiplexing is performed by using different codes for different users in each time slot.
FIG. 10 shows a concept of a shared channel with respect to a case where a shared channel held through a plurality of time slots is set in one frame. The shared channel is indicated by hatched portions in FIG. 10. Scheduling is performed for assignment of the resource to users with respect to data destined to the users. As a result of scheduling, the resource is assigned to different users in correspondence with frames. If the amount of information for a user is large, the same user may use the resource over a time period corresponding to a plurality of frames.
A CDMA system operates normally under a condition including interference and ordinarily uses transmission power control such that transmission power is increased to maintain transmission quality when the amount of interference is increased. This transmission power control is instantaneously performed to compensate for a change in reception level or a delay of transmission from a base station due to multipath fading. Interference power is generally divided into two kinds: that is, interference from other users in one cell, and interference from the outside of the cell or interference from other systems.
In a high-speed data communication (at a rate of 128 kbps or higher according to current specifications) over the above-described shared channel, the whole of one time slot or more is used for only one user. In such a high-speed communication, therefore, the above-described in-cell interference can be ignored and out-of-cell interference is dominant.
As described above, in a case where a combination of a CDMA method, a TDMA method and a TDD method is used as a wireless access method, interference power which influences the capacity for a user in a cell when the whole of one slot or more is used only for the user is only interference power form the outside of the cell. Interference power from an adjacent cell depends on power required for users in the adjacent cell and the locations of the users in the adjacent cell. Therefore, if the amount of interference from the adjacent cell is large, there is a need to increase the transmission power. Disadvantageously, the communication quality is reduced when the maximum transmission power is reached.
Further, CDMA/TDMA/TDD systems in which transmission power control is not performed have a drawback of reducing the communication quality when the amount of interference is large.